1. Field of the Invention
This invention relates to the treatment of gas streams containing nitrogen oxides; more particularly the present invention relates to a method for the treatment of such gas streams containing nitrogen oxides, particularly the tail gas from a nitric acid plant, in order to reduce or substantially eliminate the nitrogen oxides content of a gas stream while producing an aqueous ammonium nitrate solution product.
2. Description of the Prior Art
Various industrial processes, such as the production of nitric acid and the nitration of organic compounds, produce waste gas streams containing small proportions of nitrogen oxides, principally as nitric oxide and nitrogen dioxide. Discharge of such waste gas streams into the atmosphere is highly objectionable, since the nitrogen oxides are toxic, corrosive, and act as atmosphere pollutants. Because of the pollution to the atmosphere which is caused by discharge of waste gas streams containing nitrogen oxides, limits upon the nitrogen oxides content which can be present in any industrial waste gas are set by the Environmental Protection Agency. While the allowable limits, of course, differ depending upon location and other factors, such as prevailing winds, etc., it is generally necessary to treat the industrial gas streams so that the nitrogen oxides content is at least below 200 parts per million. As a result, such waste gas streams must be treated in order to eliminate nitrogen oxides prior to discharge of the waste gas stream into the atmosphere. One typical approach is to catalytically react the waste gas stream contaning nitrogen oxides with a reducing gas such as methane or hydrogen. Methane, or natural gas principally consisting of methane, is generally employed, due to economic considerations. The catalyst generally employed is an expensive platinum group metal such as platinum, palladium, rhodium, ruthenium, osmium or iridium, or a mixture of such metals. The catalyst may be in the form of unsupported wire mesh; however, the catalyst is generally supported on a suitable carrier such as alumina pellets, nichrome wire or silica gel. The catalytic reaction of the nitrogen oxides with the reducing gas results in the elimination of the nitrogen oxides, with the formation of free nitrogen and water vapor and/or carbon dioxide. When free oxygen is also present in the waste gas stream, as in the case of a tail gas from a nitric acid process, the free oxygen preferentially reacts with the reducing gas before reaction of the nitrogen oxides can take place. This results in a wasteful consumption of the reducing gas. A final waste gas stream substantially free of nitrogen oxides and suitable for discharge into the atmosphere is produced; however, the process is costly and wastes energy in the combustion process.
In addition to such catalytic reaction or catalytic combustion of the nitrogen oxides present in the industrial gas stream, a further typically employed method is to pass the nitrogen oxides containing industrial waste gas over a molecular sieve. While this method is effective to reduce the nitrogen oxides content, the successful adoption of this technique has been somewhat thwarted by the expensive nature of the technique, including the high operation costs when employing a molecular sieve. This is due to the fact that the molecular seive is quickly poisoned by contaminants within the industrial waste gas stream, making regeneration of a molecular sieve a frequent occurrence. The use of the molecular sieve to reduce the nitrogen oxides content is also disadvantageous in that there are great power requirements, thereby increasing the economic disadvantages of the procedure.
In addition to the above methods which have been proposed for the removal of nitrogen oxides from an industrial waste gas stream, various chemical methods have also been proposed. These are exemplified by the urea destruction method and a method which employs an ammonium nitrate scrubbing solution. The use of urea to remove the nitrogen oxides is exemplified by a method set forth for example in U.S. Pat. No. 3,565,575 in which a gas stream containing nitrogen oxides, such as a tail gas from a nitric acid plant, is scrubbed with an aqueous urea solution. The nitric acid reacts with the urea to form nitrogen, carbon dioxide and water. Due to the destruction of the urea in this reaction, the use of urea to remove the nitrogen oxides from an industrial waste gas stream is quite costly. As a result, this method has not been successfully practiced commercially. In addition, the removal which can be achieved by utilizing urea in this manner is not totally satisfactory.
The use of a scrubbing solution of ammonium nitrate is exemplified by U.S. Pat. No. 3,453,071. This method involves the introduction of the gas stream into an ammoniacal solution of ammonium nitrate, generally at a temperature of about 20.degree. to 30.degree. C. and an ammonium nitrate content greater than 30% by weight. An eight stage process is exemplified and applicability of a weakly acidic ammonium nitrate solution is described. A great disadvantage of this method is that to be effective, the molar ratio of NO to NO.sub.2 must be adjusted to 1:1 prior to introduction of the gas into the ammoniacal solution of ammonium nitrate. This complicates the process and makes the same economically disadvantageous. This, coupled with the fact that the removal of nitrogen oxides is apparently somewhat less than satisfactory, has lead to the conclusion that this process cannot be conducted satisfactorily on a commercial scale, absent disadvantages which cannot be removed.
An improved method and apparatus for the removal of nitrogen oxides from a gas screen, principally the tail gas from a nitric acid plant is described in applicant's copending application Ser. No. 638,043 filed Dec. 5, 1975. That co-pending application describes the removal of nitrogen oxides by sequentially contacting the nitrogen oxides-containing gas stream in gas-liquid contact apparatus with reaction liquids comprising, first an aqueous acid solution of ammonium nitrate, secondly an aqueous ammoniacal solution of ammonium nitrate and finally water, preferably slightly acidified. The apparatus which is described in connection with such method allows for co-current contact of the gas stream containing nitrogen oxides and each reaction liquid in a separate stage of the contact apparatus. At the exit of each stage, a device is provided to separate the gas from the reaction liquid so that the gas stream can be sent on for further contact or exit of the gas-liquid contact apparatus. Such device is described as preferably being a cyclone separator.
In addition, to effect an elimination of particulate matter which may be present in the gas stream, applicant's co-pending application suggests the inclusion of a venturi type orifice between the ammoniacal ammonium nitrate stage and water stage in the gas-liquid contact apparatus.
While the method and apparatus described in that copending application are extremely effective in the removal of nitrogen oxides from a gas stream to well below all current environmental standards, it has now been found that certain improvements in the NO.sub.x removal can be achieved in accordance with the present invention. For example, it has been discovered in accordance with the present invention that by operating with counter-current flow of the gas stream and reaction liquids in each stage, the need for a cyclone separator to separate gas and liquid can be eliminated. Furthermore, it has been determined that the venturi device is not necessary and simpler means for eliminating entrained particulate matter can be utilized with this counter-current flow scheme. Accordingly, the capital investment necessary to effect the NO.sub.x removal can be reduced.
In addition, it has been determined that the method described in applicant's co-pending application, specifically as it relates to co-current flow of reaction liquids and gas stream is most effective for NO.sub.x removal under high pressure conditions, i.e., pressures of 7 to 8 atmospheres in the nitric acid absorber. While such a system has applicability in low pressure environments, i.e., pressures of about 3 atmospheres in the nitric acid absorber or even atmospheric plants, it has been discovered in accordance with the present invention that the counter-current flow herein described has increased efficiency at low pressures when compared with the co-current flow of applicant's co-pending application. Accordingly, this provides a further advantage of the method described herein for the removal of NO.sub.x from a gas stream, particularly the tail gas of a nitric acid plant.